It is well known that coordination anionic polymerization using catalyst systems represented by a Ziegler-Natta catalyst allows for homopolymerization of olefins and dienes. However, it was difficult to provide efficient copolymerization of olefins and dienes using such polymerization reaction systems.
Particularly, applying a copolymer of a conjugated diene and a non-conjugated olefin to the compounded rubber results in fewer double bonds in the conjugated diene unit in the copolymer as compared with the conjugated diene polymer, and therefore, improves ozone resistance of the rubber composition. In addition, one of the characteristics other than ozone resistance required when a rubber composition is applied to various applications (such as tires, conveyor belts or anti-vibration rubber) includes good crack growth resistance.
For example, JP 2000-154210 A (PTL 1) discloses a catalyst for polymerizing conjugated dienes that contains a transition metal compound of group IV of the periodic table having cyclopentadiene ring structure, and also refers to an α-olefin such as ethylene as an exemplary monomer copolymerizable with this conjugated diene. However, PTL 1 does not provide a specific description of copolymerization of a conjugated diene compound and a non-conjugated olefin. Obviously, there is no description or suggestion of improving ozone resistance and crack growth resistance by controlling the cis content, or, by arranging a copolymer of a conjugated diene and a non-conjugated olefin so that the non-conjugated olefin is contained in an amount of 20 mol % or more and the conjugated diene has a cis-1,4 bond content of 50% or more.
For example, JP 2006-249442 A (PTL 2) discloses a catalyst for polymerizing olefins that consists of a transition metal compound such as a titanium compound and a co-catalyst, and also discloses a copolymer of an α-olefin and a conjugated diene compound. However, specific manufacture and use were ensured only if the non-conjugated olefin, α-olefin, is contained in an amount within a range of 66.7 mol % to 99.1 mol %. That is, PTL 2 does not provide any specific description or suggestion of the conjugated diene compound/non-conjugated olefin copolymer containing the non-conjugated olefin in an amount of 0 mol % to 50 mol %, nor of improving ozone resistance and crack growth resistance by controlling the cis content, or, by arranging a copolymer of a conjugated diene and a non-conjugated olefin so that the non-conjugated olefin is contained in an amount of 20 mol % or more and the conjugated diene has a cis-1,4 bond content of 50% or more.
In addition, JP 2006-503141 A (PTL 3) discloses a copolymer of ethylene and butadiene resulting from polymerizing ethylene and butadiene as starting materials using a special organic metal complex as a catalytic component. However, the copolymer of PTL 3 has a structure different from that of the copolymer of the present invention in that butadiene, which is a monomer, is inserted in the copolymer in the form of trans-1,2-cyclohexane. In addition, specific manufacture and use were ensured only if the non-conjugated olefin, ethylene, is contained in an amount within a range of 69.6 mol % to 89.0 mol %. In this case, the ethylene content was determined by 100 mol % minus the molar content of those units derived from butadiene with a known molar content. That is, PTL 3 does not also provide any specific description or suggestion of the conjugated diene compound/non-conjugated olefin copolymer containing the non-conjugated olefin in an amount of 0 mol % to 50 mol %, nor of improving ozone resistance and crack growth resistance by controlling the cis content, or, by arranging a copolymer of a conjugated diene and a non-conjugated olefin so that the non-conjugated olefin is contained in an amount of 20 mol % or more and the conjugated diene has a cis-1,4 bond content of 50% or more.